The San Francisco 49ers have hired Jim O'Neil as defensive coordinator.

O'Neil, 37, takes over the position that Eric Mangini held on Jim Tomsula's staff last season. The 49ers ranked 29th in the NFL in yards allowed last season and 18th in the league in points allowed at 24.2 points per game.

O'Neil spent the past two seasons with the Cleveland Browns as defensive coordinator.

Rival defensive coaches believe Chip Kelly can once again make Colin Kaepernick one of the NFL's most dangerous quarterbacks.

"I think [Kaepernick] is a good enough passer, but obviously what'll be a nightmare is his ability to run," on defensive coordinator said. "That offense is straight 'Freddy Krueger' when you have a quarterback that can pull the ball and run at any given time."

Kelly's offense stalled with the Philadelphia Eagles when he didn't have a quarterback capable of pulling off the second part of the zone read.

"The fact that Kap can make guys miss and get in the open field, they didn't have that last year at all in Philly," another defensive coordinator said. "We treated [Sam] Bradford like he was under center. There was zero threat of him running the ball. We told our guys, 'Don't treat him like he's in the shotgun, he's never gonna pull the ball.' "

The San Francisco 49ers have offered the offensive coordinator job to Curtis Modkins.

Modkins is currently the Detroit Lions' run game coordinator.

Chip Kelly typically has a hands-on approach to running his offense.

Eric Mangini has been fired as defensive coordinator of the San Francisco 49ers.

Mangini had been with the 49ers organization for three seasons, but was the defensive coordinator for just one year. Mangini worked as a tight ends coach and offensive consultant under Jim Harbaugh in 2014 and 2013, respectively.

Mangini was previously a head coach with the New York Jets and Cleveland Browns.

Jason Tarver is a possible replacement for Mangini.

Mike Vrabel has turned down an offer to become the San Francisco 49ers' defensive coordinator.

Vrabel has been linebackers coach of the Houston Texans over the past two seasons.

Bill O'Brien and general manager Rick Smith worked hard behind the scenes to keep Vrabel and will give him a new contract, the Houston Chronicle reported, citing sources.

Vrabel met with new 49ers coach Chip Kelly on Tuesday before declining the job, the Chronicle reported.

Chip Kelly declined to reveal if he favors Colin Kaepernick or Blaine Gabbert when he was introduced to the media as head coach of the San Francisco 49ers.

"Obviously Kap is an extremely talented football player and you need a good quarterback to win," Kelly said. "But I was also impressed from the film I watched in terms of how Blaine played this year. Both of those players made this an attractive situation."

Kelly said league rules prevent him from discussing any football schematics with either Kaepernick or Gabbert until the offseason program begins April 4th.

Kaepernick's contract becomes guaranteed if he's on the roster on April 1st.

Kelly disputed reports that he was interested in trading a year ago for Kaepernick with the Philadelphia Eagles.

"I don't know where that report came from but we were very happy with Sam (Bradford) in Philadelphia," Kelly said. "In terms of Kap, he was very difficult for us to play against on this field two years ago. I haven't done a ton of tape study on how he played (last year). We're in that process right now."

Trent Baalke called hiring Chip Kelly an easy decision for the San Francisco 49ers.

"At the end of the day, this was a fairly easy decision," Baalke said about hiring Kelly. "It was evident early on in our conversations with Chip that he shared a very similar vision to us with respect to the direction of this football team. He's a well-rounded, highly innovative coach with a proven track record of success at every level of competition, including the NFL.

"He's a strong leader of men and somebody I really consider a true football guy."

A reporter asked if Kelly had plans to make changes to his up-tempo offense, which seemed to lose efficiency as his Eagles tenure wore on:

"I think we're receptive to anything that's going to help us win football games and I know the difficult of what we do offensively does to defenses. We'll look at anything."

LeSean McCoy was asked for offer advice to the San Francisco 49ers' players on how to best adapt to Chip Kelly.

McCoy played for Kelly for two seasons with the Philadelphia Eagles before he was traded to the Buffalo Bills.

"Every situation is different," McCoy said. "The one thing I will say about Chip is that he wants to win. He's very, very intelligent. The offense, they'll find ways to get the big numbers. They'll find ways to put the stats up offensively. With him coming in there, no matter how good of a player or low player you are, if you just automatically believe him -- no matter if it sounds crazy, you're like what is he talking about -- if you just automatically believe and buy in, I think you'll be fine."

The San Francisco 49ers fired 10 assistants on Tuesday, including offensive coordinator Geep Chryst.

Eric Mangini, who was defensive coordinator last season, remains employed so far. 

The 49ers are expected to interview Mike Vrabel for the defensive coordinator job.

Chip Kelly's contract with the San Francisco 49ers is a four-year, $24 million deal.

Kelly becomes the highest paid coach in 49ers' history.

The 49ers also owe Jim Tomsula's $2.5 million salary for the next three seasons.

Jim Harbaugh signed a five-year, $25 million contract in January 2011. 

Harbaugh and the 49ers had talks on an extension after the 2013 and 2014 seasons.

Kelly left Oregon in 2013 to sign a reported five-year, $32.5 million contract with the Philadelphia Eagles that made him one of the league’s highest-paid coaches.